a) Field of the Invention
The present invention relates to an improved iron ore sinter for use in a blast furnace. More particularly, it relates to an improved iron ore sinter wherein the improvement consists in using synthetic olivine in place of natural olivine or dolomite as a source of MgO.
b) Brief Description of the Prior Art
As is well known in the art of metallurgy, four basic ingredients have to be fed into a blast furnace to produce iron by chemical reduction of iron oxides and/or other iron-bearing substances, namely:
a) the iron oxides and/or iron-bearing substances per se, in the form of sinters, pellets, briquettes or any other type of agglomerates, or occasionally lumpy raw ores;
b) basic fluxes including a source of CaO and a source of MgO selected amongst for example, limestone, dolomite, natural olivine and the like, whose purpose is to form a slag by reaction with the acid gangue constituents of the feed;
c) metallurgical coke used as a heat-generating combustible and as a reducing agent when it is transformed into carbon monoxide by controlled combustion with air; and
d) air to provide oxygen and thus support the combustion and slag formation.
All of these basic ingredients may be fed into the blast furnace one at a time, in predetermined amounts, to form successive layers of iron oxides, fluxes and coke through which air is blown. As the coke burns, the iron oxides or other iron-bearing substances melt and are reduced to form the desired iron in molten form. The impurities are "collected" in the liquid slag formed by the fluxes and can be separated from the iron and removed from the furnace.
In recent years, it has been suggested to combine all of these ingredients together in the form of agglomerates, especially pellets or sinters, in order to improve the permeability of the charge and thus permit higher gas flow and better gas-solid contact within the furnace. In this connection, reference can be made, by way of example, to U.S. Pat. No. 4,518,428 issued in 1985 to International Minerals & Chemical Corp., or U.S. Pat. No. 4,657,584 issued in 1987 to U.S. Steel Corp.
The main advantage of using pellets or sinters in which all the basic ingredients are combined (except air) is that such a use substantially reduces, not to say eliminates the introduction of basic fluxes in raw form into the furnace. As a result:
1) substantial savings are obtained in the consumption of expensive metallurgical coke, which would otherwise be required to calcine the raw fluxes, and
2) blast furnace productivity (expressed in tons/m.sup.2 of hearth area) is increased by as much as 50%.
As already indicated hereinabove, the fluxes used in the blast furnace must include a source of CaO and a source of MgO. In operation, both of these oxides react with the acid gangue usually found in the iron-bearing substances used as an iron source, which gangue includes SiO.sub.2, Al.sub.2 O.sub.3 and other impurities such as sulphur and phosphorous, the product of this reaction being the slag.
In practice, the formation of a slag of proper chemistry and fluidity is of a great importance to activate smooth operation of the blast furnace. Indeed, the volume and chemistry of the slag whose purpose is to carry the unwanted impurities and help in the separation of iron in the hearth of the furnace and subsequent removal of this iron from the furnace are both known to influence the thermal balance and the partition of sulphur between the slag and the molten iron.
The major chemical constituents and composition of the slags of most of the existing blast furnaces presently in operation, are as follows:
TABLE I ______________________________________ % CaO % MgO % Al.sub.2 O.sub.3 % SiO.sub.2 ______________________________________ 34-47 4-12 10-22 31-39 ______________________________________
As can be seen, MgO is an important ingredient of the slag.
In practice, when use is made of iron ore sinters, the MgO found in the slag comes from the sinter into which the basic fluxes are incorporated. Wherever necessary, but to a lesser extent, additional MgO may be introduced in the form of fluxed pellets or through direct addition of dolomite, natural olivine (see U.S. Pat. No. 4,518,428), periclase (see U.S. Pat. No. 4,657,584) or similar material.
As already indicated thereinabove, the practice of adding an MgO-containing material directly into the blast furnace has largely been discontinued because of economic and metallurgical considerations. Therefore, the iron-bearing agglomerates that are presently used in the form of sinters and pellets invariably contain certain amounts of MgO, which usually vary between 1 and 3% by weight and in special cases, up to 10%.
The incorporation of MgO directly into the agglomerates (sinters or pellets) has many advantages, some of which are:
improved resistance to low-temperature degradation, leading to a decrease in flue dust losses;
improved high-temperature reduction characteristics, maintaining the structure of agglomerates for good reduction;
reduction in the range of softening and meltdown temperature and increase in the respective temperatures;
reduction of hanging and scaffolding;
good desulphurizing properties and strong affinity for sulphur;
minimization of Fe loss in the slag; and
for high aluminous blast furnace slags, increase in the slag fluidity.
In essence, the incorporation of MgO in agglomerates such as sinters or pellets, leads to smooth and economic blast furnace operation and improved hot metal quality.
Presently, the MgO incorporated into the agglomerates comes from dolomite, natural olivine, dunite, burnt dolomite etc. The use of such "natural" materials as sources of MgO is dictated primarily by cost, quality, and proximity to the source, despite the fact that the quality of sinter may substantially vary depending on the source of the MgO-containing material. It has been found however in many European and Australian steel plants, that the use of natural olivine as a source of MgO is better than the use of any other material from the standpoint of productivity and as well as quality of the agglomerates. The use of natural olivine is suggested in U.S. Pat. No. 4,518,428 but is rather limited in North America because of its non-availability and high importation cost, although it is admitted that the addition of natural olivine to the blast furnace increases the MgO content of the slag and the fluidity range of the slag, and makes it less sensitive to other chemical impurities or to temperature variation.
On the other hand, it is also known that the viscosity of the slag is dependent on the basicity ratio. The basicity ratio of the sinter, (CaO+MgO) to (SiO.sub.2 +Al.sub.2 O.sub.3), should remain preferably between 1.5 and 2.6. Since olivine is a mineral of general formula Mg.sub.2 SiO.sub.4, one can see that the addition of olivine as a source of MgO in a blast furnace is particularly interesting since, with such a mineral, SiO.sub.2 is added at the same rate as MgO in the furnace, thereby leaving the basicity ratio substantially unaffected.
In practice, olivine added to the blast furnace as a "trim", is of the same size as the other raw materials, i.e. 10-50 mm with less than 10% of the particles below 10 mm. A good lump size is important in the blast furnace where permeability must be maintained in order to prevent poor gas flow and the build up of back pressure. In turn, good permeability is advantageous to ensure a continuous blast of gas which results in an efficient furnace operation with the attendant reduction in coke rate (volume of coke required per ton of hot metal being produced).
Dolomite, extensively used in the past as a source of MgO, is steadily decreasing in popularity because, on the one hand, it requires the addition of silica to maintain the basicity ratio of the slag and, on the other hand, it must be calcined prior to being used.
Therefore, the use of lump olivine as a trim represents a less costly single step procedure, provided that the mineral is readily available.